1. Field of the invention
The present invention relates to a brake booster and in particular to a booster to be connected to a master cylinder of a vehicle brake system.
2. Description of the Related Art
Japanese Utility Model Laid-Open Patent No. 60-30871, published without examination in 1985, discloses a booster having a housing in which a constant-pressure chamber and a variable-pressure chamber are defined by a power piston. An input rod receives an input force from a brake pedal and is operatively connected to one end of the power piston. A valve is provided in the power piston and is operatively connected to the input rod so as to open upon receipt of the input force in such a manner that a differential pressure between the chambers moves the power piston. An output rod is operatively connected to the other end of the power piston for actuating a master cylinder.
In the above-described booster, when a driver of the vehicle depresses a brake pedal, an input force F1 is applied to the input rod. The force is increased due to the well-known function of the booster and a resulting force is transmitted as an output force F2 of the booster to the master cylinder for the actuation thereof via the output rod.
However, in the above-described booster, the output force F2 is increased to a greater degree than that of the input force F1 (cf. FIG. 2). Accordingly, if the input force F1 is set at a predetermined value, the output force F2 is also at a value corresponding to the value of the force F1. Accordingly, an insufficient braking force is felt by the driver due to a decrease in the coefficient of friction (.mu.) of a brake-pad against a rotor or drum.